Chemogenetic Modulation of Orexin Neurons Reverses Changes in Anxiety and Locomotor Activity in the A53T Mouse Model of Parkinson's Disease

Multidimensional mechanisms of anxiety and depression in Parkinson's disease: Integrating neuroimaging, neurocircuits, and molecular pathways

Anxiety and depression are common non-motor symptoms of Parkinson's disease (PD) that significantly affect patients' quality of life. In recent years, our understanding of PD has advanced through multifaceted studies on the pathological mechanisms associated with anxiety and depression in PD. These classic psychiatric symptoms involve complex pathophysiology, with both distinct features and connections to the mechanisms underlying the aetiology of PD. Furthermore, the co-occurrence of anxiety and depression in PD blurs the boundaries between them. Therefore, a comprehensive summary of the pathogenic mechanisms associated with anxiety and depression will aid in better addressing the emergence of these classic psychiatric symptoms in PD. This article integrates neuroanatomical, neural projection, neurotransmitter, neuroinflammatory, brain-gut axis, neurotrophic, hypothalamic-pituitary-adrenal axis, and genetic perspectives to provide a comprehensive description of the core pathological alterations underlying anxiety and depression in PD, aiming to provide an up-to-date perspective and broader therapeutic prospects for PD patients suffering from anxiety or depression.

Attention-deficit/hyperactivity disorder-related psychomotor activity and altered neuronal activity in the medial prefrontal cortex and striatum in the A53T mouse model of Parkinson's disease and other synucleinopathies: Findings from an "endophenotype" approach

Attention-Deficit/Hyperactivity Disorder (ADHD) is associated with an increased risk of Parkinson's disease (PD) and other synucleinopathies later in life. The severity of the ADHD phenotype may play a significant role in this association. There is no indication that any of the existing animal models can unify these disorders. Using the Open Field Test, amphetamine-challenge test, Western blot and immunohistochemical analysis of neuronal activity markers (c-Fos, FosB and ΔFosB) we performed a deliberate neurobehavioral characterization of 6-month-old hemizygous A53T carriers (A53T+) of the JAX006823 strain, evaluating the utility of this transgenic mouse model of PD and other synucleinopathies in ADHD/PD continuum research. Adhering to the "endophenotype" approach, non-transgenic littermates (A53T-) and C57BL/6J mice (used to maintain the colony) were examined with A53T+ mice, to differentiate between biomarkers of transgenicity and endophenotypic traits related to the genetic background of the strain. Obtained results revealed that increased behavioral and acute striatal response to novelty, increased basal neuronal activity of the ventromedial prefrontal cortex and rate-dependent calming effect of amphetamine were endophenotypic characteristics of the strain. Increased acute response of the medial prefrontal cortex to novelty and chronic increase in neuronal activity of the striatum appeared as the mark of transgenicity. To the best of our knowledge, this is the first study to indicate external validity of a transgenic mouse model of PD and other synucleinopathies with the neurobehavioral pathology associated with ADHD, hinting at its potential in preclinical research of ADHD/PD continuum. The full capacity of the model remains to be explored.

Sex-dimorphic functions of orexin in neuropsychiatric disorders

The orexin system regulates a variety of physiological functions, including the sleep-wake cycle, addiction, foraging behavior, stress and cognitive functioning. Orexin levels in central and peripheral are related to the pathogenesis of many diseases, most notably the narcolepsy, eating disorders, stress-related psychiatric disorders, and neurodegenerative diseases. Recently, it has been reported that the orexin system is distinctly sexually dimorphic, and is strongly associated with neuropsychiatric disorders. In this review, we analyzed advancements in the sex differences in the orexin system and their connection to psychoneurological conditions. Considering the scarcity of research in this domain, more research is imperative to reveal the underlying mechanisms.

The wake- and sleep-modulating neurons of the lateral hypothalamic area demonstrate a differential pattern of degeneration in Alzheimers disease

BACKGROUND

Sleep-wake dysfunction is an early and common event in Alzheimer's disease (AD). The lateral hypothalamic area (LHA) regulates the sleep and wake cycle through wake-promoting orexinergic neurons (OrxN) and sleep-promoting melanin-concentrating hormone or MCHergic neurons (MCHN). These neurons share close anatomical proximity with functional reciprocity. This study investigated LHA OrxN and MCHN loss patterns in AD individuals. Understanding the degeneration pattern of these neurons will be instrumental in designing potential therapeutics to slow down the disease progression and remediate the sleep-wake dysfunction in AD.

METHODS

Postmortem human brain tissue from donors with AD (across progressive stages) and controls were examined using unbiased stereology. Formalin-fixed, celloidin-embedded hypothalamic sections were stained with Orx-A/MCH, p-tau (CP13), and counterstained with gallocyanin. Orx or MCH-positive neurons with or without CP13 inclusions and gallocyanin-stained neurons were considered for stereology counting. Additionally, we extracted RNA from the LHA using conventional techniques. We used customized Neuropathology and Glia nCounter (Nanostring) panels to study gene expression. Wald statistical test was used to compare the groups, and the genes were considered differentially expressed when the p-value was <.05.

RESULTS

We observed a progressive decline in OrxN alongside a relative preservation of MCHN. OrxN decreased by 58% (p=0.03) by Braak stages (BB) 1-2 and further declined to 81% (p=0.03) by BB 5-6. Conversely, MCHN demonstrated a non-statistical significant decline (27%, p=0.1088) by BB 6. We observed a progressive increase in differentially expressed genes (DEGs), starting with glial profile changes in BB2. While OrxN loss was observed, Orx-related genes showed upregulation in BB 3-4 compared to BB 0-1. GO and KEGG terms related to neuroinflammatory pathways were mainly enriched.

CONCLUSIONS

To date, OrxN loss in the LHA represents the first neuronal population to die preceding the loss of LC neurons. Conversely, MCHN shows resilience to AD p-tau accumulation across Braak stages. The initial loss of OrxN correlates with specific neuroinflammation, glial profile changes, and an overexpression of HCRT, possibly due to hyperexcitation following compensation mechanisms. Interventions preventing OrxN loss and inhibiting p-tau accumulation in the LHA could prevent neuronal loss in AD and, perhaps, the progression of the disease.

Interactions between Lateral Hypothalamic Orexin and Dorsal Raphe Circuitry in Energy Balance

Orexin/hypocretin terminals innervate the dorsal raphe nucleus (DRN), which projects to motor control areas important for spontaneous physical activity (SPA) and energy expenditure (EE). Orexin receptors are expressed in the DRN, and obesity-resistant (OR) rats show higher expression of these receptors in the DRN and elevated SPA/EE. We hypothesized that orexin-A in the DRN enhances SPA/EE and that DRN-GABA modulates the effect of orexin-A on SPA/EE. We manipulated orexin tone in the DRN either through direct injection of orexin-A or through the chemogenetic activation of lateral-hypothalamic (LH) orexin neurons. In the orexin neuron activation experiment, fifteen minutes prior to the chemogenetic activation of orexin neurons, the mice received either the GABA-agonist muscimol or antagonist bicuculline injected into the DRN, and SPA/EE was monitored for 24 h. In a separate experiment, orexin-A was injected into the DRN to study the direct effect of DRN orexin on SPA/EE. We found that the activation of orexin neurons elevates SPA/EE, and manipulation of GABA in the DRN does not alter the SPA response to orexin neuron activation. Similarly, intra-DRN orexin-A enhanced SPA and EE in the mice. These results suggest that orexin-A in the DRN facilitates negative energy balance by increasing physical activity-induced EE, and that modulation of DRN orexin-A is a potential strategy to promote SPA and EE.

Anxiety in synucleinopathies: neuronal circuitry, underlying pathomechanisms and current therapeutic strategies

Synucleinopathies are neurodegenerative disorders characterized by alpha-synuclein (αSyn) accumulation in neurons or glial cells, including Parkinson's disease (PD), dementia with Lewy bodies (DLB), and multiple system atrophy (MSA). αSyn-related pathology plays a critical role in the pathogenesis of synucleinopathies leading to the progressive loss of neuronal populations in specific brain regions and the development of motor and non-motor symptoms. Anxiety is among the most frequent non-motor symptoms in patients with PD, but it remains underrecognized and undertreated, which significantly reduces the quality of life for patients. Anxiety is defined as a neuropsychiatric complication with characteristics such as nervousness, loss of concentration, and sweating due to the anticipation of impending danger. In patients with PD, neuropathology in the amygdala, a central region in the anxiety and fear circuitry, may contribute to the high prevalence of anxiety. Studies in animal models reported αSyn pathology in the amygdala together with alteration of anxiety or fear learning response. Therefore, understanding the progression, extent, and specifics of pathology in the anxiety and fear circuitry in synucleinopathies will suggest novel approaches to the diagnosis and treatment of neuropsychiatric symptoms. Here, we provide an overview of studies that address neuropsychiatric symptoms in synucleinopathies. We offer insights into anxiety and fear circuitry in animal models and the current implications for therapeutic intervention. In summary, it is apparent that anxiety is not a bystander symptom in these disorders but reflects early pathogenic mechanisms in the cortico-limbic system which may even contribute as a driver to disease progression.

Several neuropeptides involved in parkinsonian neuroprotection modulate the firing properties of nigral dopaminergic neurons

Parkinson's disease is characterized by progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta. The surviving nigral dopaminergic neurons display altered spontaneous firing activity in Parkinson's disease. The firing rate of nigral dopaminergic neurons decreases long before complete neuronal death and the appearance of parkinsonian symptoms. A mild stimulation could rescue dopaminergic neurons from death and in turn play neuroprotective effects. Several neuropeptides, including cholecystokinin (CCK), ghrelin, neurotensin, orexin, tachykinins and apelin, within the substantia nigra pars compacta play important roles in the modulation of spontaneous firing activity of dopaminergic neurons and therefore involve motor control and motor disorders. Here, we review neuropeptide-induced modulation of the firing properties of nigral dopaminergic neurons. This review may provide a background to guide further investigations into the involvement of neuropeptides in movement control by modulating firing activity of nigral dopaminergic neurons in Parkinson's disease.

Effects of Lanthionine Ketimine-5-Ethyl Ester on the α-Synucleinopathy Mouse Model

Potentially druggable mechanisms underlying synaptic deficits seen in Parkinson's disease (PD) and dementia with Lewy bodies (DLB) are under intense interrogations. In addition to defective synaptic vesicle trafficking, cytoskeletal disruption, autophagic perturbation, and neuroinflammation, hyperphosphorylation of microtubule-associated protein collapsin response mediator protein 2 (CRMP2, also known as DPYSL2) is newly determined to correlate with synaptic deficits in human DLB. The small molecule experimental therapeutic, lanthionine ketimine-5-ethyl ester (LKE), appears to interact with CRMP2 in a host of neurodegenerative mouse models, normalizing its phosphorylation level while promoting healthful autophagy in cell culture models and suppressing the proinflammatory phenotype of activated microglia. Accordingly, this study examined the effect of LKE on α-synuclein A53T transgenic (Tg) mice which were employed as a DLB model. We found that chronic administration of LKE to A53T mice suppressed (1) the accumulation of LBs, (2) neuroinflammatory activation of microglia, (3) impairment of contextual fear memory, and (4) CRMP2 phosphorylation at Thr509 in A53T Tg mice. These results suggest that CRMP2 phosphorylation by GSK3β in the hippocampus is related to pathology and memory impairment in DLB, and LKE may have clinical implications in the treatment of α-synucleinopathy.

Orexin enhances neuronal synchronization in adult rat hypothalamic culture: a model to study hypothalamic function

The regulation of sleep/wake behavior and energy homeostasis is maintained in part by the hypothalamic neuropeptide orexin A (OXA, hypocretin). Reduction in orexin signaling is associated with sleep disorders and obesity, whereas higher lateral hypothalamic (LH) orexin signaling and sensitivity promotes obesity resistance. Similarly, dysregulation of hypothalamic neural networks is associated with onset of age-related diseases, including obesity and several neurological diseases. Despite the association of obesity and aging, and that adult populations are the target for the majority of pharmaceutical and obesity studies, conventional models for neuronal networks utilize embryonic neural cultures rather than adult neurons. Synchronous activity describes correlated changes in neuronal activity between neurons and is a feature of normal brain function, and is a measure of functional connectivity and final output from a given neural structure. Earlier studies show alterations in hypothalamic synchronicity following behavioral perturbations in embryonic neurons obtained from obesity-resistant rats and following application of orexin onto embryonic hypothalamic cultures. Synchronous network dynamics in adult hypothalamic neurons remain largely undescribed. To address this, we established an adult rat hypothalamic culture in multi-electrode-array (MEA) dishes and recorded the field potentials. Then we studied the effect of exogenous orexin on network synchronization of these adult hypothalamic cultures. In addition, we studied the wake promoting effects of orexin in vivo when directly injected into the lateral hypothalamus (LH). Our results showed that the adult hypothalamic cultures are viable for nearly 3 mo in vitro, good quality MEA recordings can be obtained from these cultures in vitro, and finally, that cultured adult hypothalamus is responsive to orexin. These results support that adult rat hypothalamic cultures could be used as a model to study the neural mechanisms underlying obesity. In addition, LH administration of OXA enhanced wakefulness in rats, indicating that OXA enhances wakefulness partly by promoting neural synchrony in the hypothalamus.NEW & NOTEWORTHY This study, for the first time, demonstrates that adult hypothalamic cultures are viable in vitro for a prolonged duration and are electrophysiologically active. In addition, the study shows that orexin enhances neural synchronization in adult hypothalamic cultures.

Parkinson disease: Protective role and function of neuropeptides

Neuropeptides are bioactive molecules, made up of small chains of amino acids, with many neuromodulatory properties. Several lines of evidence suggest that neuropeptides, mainly expressed in the central nervous system (CNS), play an important role in the onset of Parkinson's Disease (PD) pathology. The wide spread disruption of neuropeptides has been excessively demonstrated to be related to the pathophysiological symptoms in PD where impairment in motor function per example was correlated with neuropeptides dysregulation in the substantia niagra (SN). Moreover, the levels of different neuropeptides have been found modified in the cerebrospinal fluid and blood of PD patients, indicating their potential role in the manifestation of PD symptoms and dysfunctions. In this review, we outlined the neuroprotective effects of neuropeptides on dopaminergic neuronal loss, oxidative stress and neuroinflammation in several models and tissues of PD. Our main focus was to elaborate the role of orexin, pituitary adenylate cyclase activating polypeptide (PACAP), vasoactive intestinal peptide (VIP), opioids, angiotensin, carnosine and many others in the protection and/or involvement in the neurodegeneration of striatal dopaminergic cells. Further studies are required to better assess the mode of action and cellular mechanisms of neuropeptides in order to shift the focus from the in vitro and in vivo testing to applicable clinical testing. This review, allows a support for future use of neuropeptides as therapeutic solution for PA pathophysiology.

Orexin, serotonin, and energy balance

The lateral hypothalamus is critical for the control of ingestive behavior and spontaneous physical activity (SPA), as lesion or stimulation of this region alters these behaviors. Evidence points to lateral hypothalamic orexin neurons as modulators of feeding and SPA. These neurons affect a broad range of systems, and project to multiple brain regions such as the dorsal raphe nucleus, which contains serotoninergic neurons (DRN) important to energy homeostasis. Physical activity is comprised of intentional exercise and SPA. These are opposite ends of a continuum of physical activity intensity and structure. Non‐goal‐oriented behaviors, such as fidgeting, standing, and ambulating, constitute SPA in humans, and reflect a propensity for activity separate from intentional activity, such as high‐intensity voluntary exercise. In animals, SPA is activity not influenced by rewards such as food or a running wheel. Spontaneous physical activity in humans and animals burns calories and could theoretically be manipulated pharmacologically to expend calories and protect against obesity. The DRN neurons receive orexin inputs, and project heavily onto cortical and subcortical areas involved in movement, feeding and energy expenditure (EE). This review discusses the function of hypothalamic orexin in energy‐homeostasis, the interaction with DRN serotonin neurons, and the role of this orexin‐serotonin axis in regulating food intake, SPA, and EE. In addition, we discuss possible brain areas involved in orexin–serotonin cross‐talk; the role of serotonin receptors, transporters and uptake‐inhibitors in the pathogenesis and treatment of obesity; animal models of obesity with impaired serotonin‐function; single‐nucleotide polymorphisms in the serotonin system and obesity; and future directions in the orexin–serotonin field.

This article is categorized under:

Metabolic Diseases > Molecular and Cellular Physiology

Orexin-A in Patients With Lewy Body Disease: A Systematic Review and Meta-Analysis

Background

Abnormal orexin-A levels in cerebrospinal fluid (CSF) have been identified in Alzheimer's disease (AD) and other neurodegenerative diseases. However, few studies have focused on Lewy body disease (LBD) and often with debatable outcomes. Thus, we performed this systematic review and meta-analysis to investigate orexin-A levels in LBD by incorporating data from different studies.

Methods

We gathered studies comparing orexin-A levels in patients with LBD and controls (including healthy controls and other dementia subtypes). In the initial search, 117 relevant articles were identified. After a selection process, seven studies, conducted in Japan, USA, Spain, Switzerland, France, Italy, and Netherlands, were chosen.

Results

In total, 179 patients with LBD and 253 controls were included. Patients with LBD had significantly lower mean orexin-A CSF levels when compared with patients with AD [standard mean difference (SMD): -0.35, 95% confidence interval (CI): -0.70 to -0.00, Z = 1.96, P = 0.05], whereas mean orexin-A levels were significantly higher when compared with patients with frontotemporal lobar degeneration (FTLD) (SMD: 0.61, 95% CI: 0.23-0.99, Z = 3.12, P = 0.002). Orexin-A CSF levels in LBD patients were approximately equal to levels in healthy elderly individuals, whereas they were significantly decreased in LBD patients with excessive daytime sleepiness (EDS) (SMD: -0.15, 95% CI: -0.59 to 0.29, Z = 0.67, P = 0.50).

Conclusions

We showed that orexin-A levels in patients with LBD were not very different from those in normal elderly individuals, whereas they were lower than those in AD patients and higher than those in FTLD patients. The influence of hypersomnia on orexin-A levels should be carefully interpreted.

Systematic Review Registration

https://www.crd.york.ac.uk/prospero/, identifier CRD42021265900.

Alpha-synuclein pathology, microgliosis, and parvalbumin neuron loss in the amygdala associated with enhanced fear in the Thy1-aSyn model of Parkinson's disease

In Parkinson's disease (PD), the second most common neurodegenerative disorder, non-motor symptoms often precede the development of debilitating motor symptoms and present a severe impact on the quality of life. Lewy bodies containing misfolded α-synuclein progressively develop in neurons throughout the peripheral and central nervous system, which may be correlated with the early development of non-motor symptoms. Among those, increased fear and anxiety is frequent in PD and thought to result from pathology outside the dopaminergic system, which has been the focus of symptomatic treatment to alleviate motor symptoms. Alpha-synuclein accumulation has been reported in the amygdala of PD patients, a brain region critically involved in fear and anxiety. Here we asked whether α-synuclein overexpression alone is sufficient to induce an enhanced fear phenotype in vivo and which pathological mechanisms are involved. Transgenic mice expressing human wild-type α-synuclein (Thy1-aSyn), a well-established model of PD, were subjected to fear conditioning followed by extinction and then tested for extinction memory retention followed by histopathological analysis. Thy1-aSyn mice showed enhanced tone fear across acquisition and extinction compared to wild-type littermates, as well as a trend to less retention of fear extinction. Immunohistochemical analysis of the basolateral nucleus of the amygdala, a nucleus critically involved in tone fear learning, revealed extensive α-synuclein pathology, with accumulation, phosphorylation, and aggregation of α-synuclein in transgenic mice. This pathology was accompanied by microgliosis and parvalbumin neuron loss in this nucleus, which could explain the enhanced fear phenotype. Importantly, this non-motor phenotype was detected in the pre-clinical phase, prior to dopamine loss in Thy1-aSyn mice, thus replicating observations in patients. Results obtained in this study suggest a possible mechanism by which increased anxiety and maladaptive fear processing may occur in PD, opening a door for therapeutic options and further early biomarker research.

Biosensors in Parkinson's disease

Parkinson's disease (PD) is one of the most critical disorders of the elderly and strongly associated with increased disability, and reduced quality of life. PD is a progressive neurodegenerative disease affecting more than six million people worldwide. Evaluation of clinical manifestations, as well as movement disorders by a neurologist and some routine laboratory tests are the most important diagnostic methods for PD. However, routine and old methods have several disadvantages and limitations such as low sensitivity and selectivity, high cost, and need for advanced equipment. Biosensors technology opens up new diagnoses approach for PD with the use of a new platform that allows reliable, repeatable, and multidimensional identification to be made with minimal problem and discomfort for patients. For instance, biosensing systems can provide promising tools for PD treatment and monitoring. Amongst biosensor technology, electrochemical techniques have been at the frontline of this progress, thanks to the developments in material science, such as gold nanoparticles (AuNPs), quantum dots (QDs), and carbon nanotubes (CNTs). This paper evaluates the latest progress in electrochemical and optical biosensors for PD diagnosis.

Content of Peripheral Blood T- and B-Cell Subpopulations in Transgenic A53T Mice of Different Age (A Model of Parkinson's Disease)

We analyzed the behavior and peripheral blood T- and B-cell subpopulations in mice overexpressing the mutant form of human α-synuclein (A53T) in comparison with mice of the wild type (WT) parent C57BL/6J strain. Behavioral phenotype and the content of various cell subpopulations of A53T mice depended on animal age. Young (2-month-old mice) were characterized by low emotionality and the most pronounced changes in cell subpopulation composition (an increase in CD3⁺T cells and CD4⁺T helper cells, a decrease in CD19⁺B cells along with unchanged content of CD3⁺CD4⁺CD25⁺T-regulatory cells and CD19⁺CD25⁺B-regulatory cells). In old A53T mice (10-month-old), movement impairments appeared and increased numbers of CD4⁺T helper cells and CD3⁺CD4⁺CD25⁺T-regulatory cells were revealed.

Inhibition of Orexin/Hypocretin Neurons Ameliorates Elevated Physical Activity and Energy Expenditure in the A53T Mouse Model of Parkinson's Disease

Aside from the classical motor symptoms, Parkinson's disease also has various non-classical symptoms. Interestingly, orexin neurons, involved in the regulation of exploratory locomotion, spontaneous physical activity, and energy expenditure, are affected in Parkinson's. In this study, we hypothesized that Parkinson's-disease-associated pathology affects orexin neurons and therefore impairs functions they regulate. To test this, we used a transgenic animal model of Parkinson's, the A53T mouse. We measured body composition, exploratory locomotion, spontaneous physical activity, and energy expenditure. Further, we assessed alpha-synuclein accumulation, inflammation, and astrogliosis. Finally, we hypothesized that chemogenetic inhibition of orexin neurons would ameliorate observed impairments in the A53T mice. We showed that aging in A53T mice was accompanied by reductions in fat mass and increases in exploratory locomotion, spontaneous physical activity, and energy expenditure. We detected the presence of alpha-synuclein accumulations in orexin neurons, increased astrogliosis, and microglial activation. Moreover, loss of inhibitory pre-synaptic terminals and a reduced number of orexin cells were observed in A53T mice. As hypothesized, this chemogenetic intervention mitigated the behavioral disturbances induced by Parkinson's disease pathology. This study implicates the involvement of orexin in early Parkinson's-disease-associated impairment of hypothalamic-regulated physiological functions and highlights the importance of orexin neurons in Parkinson's disease symptomology.

Does chronic systemic injection of the DREADD agonists clozapine-N-oxide or Compound 21 change behavior relevant to locomotion, exploration, anxiety, and depression in male non-DREADD-expressing mice?

Designer Receptors Exclusively Activated by Designer Drugs (DREADDs) are chemogenetic tools commonly-used to manipulate brain activity. The most widely-used synthetic DREADD ligand, clozapine-N-oxide (CNO), is back-metabolized to clozapine which can itself activate endogenous receptors. Studies in non-DREADD-expressing rodents suggest CNO or a DREADD agonist that lacks active metabolites, such as Compound 21 (C21), change rodent behavior (e.g. decrease locomotion), but chronic injection of CNO does not change locomotion. However, it is unknown if chronic CNO changes behaviors relevant to locomotion, exploration, anxiety, and depression, or if chronic C21 changes any aspect of mouse behavior. Here non-DREADD-expressing mice received i.p. Vehicle (Veh), CNO, or C21 (1 mg/kg) 5 days/week for 16 weeks and behaviors were assessed over time. Veh, CNO, and C21 mice had similar weight gain over the 16-week-experiment. During the 3rd injection week, CNO and C21 mice explored more than Veh mice in a novel context and had more open field center entries; however, groups were similar in other measures of locomotion and anxiety. During the 14th-16th injection weeks, Veh, CNO, and C21 mice had similar locomotion and anxiety-like behaviors. We interpret these data as showing chronic Veh, CNO, and C21 injections given to male non-DREADD-expressing mice largely lack behavioral effects. These data may be helpful for behavioral neuroscientists when study design requires repeated injection of these DREADD agonists.

Orexin and Parkinson's disease: A protective neuropeptide with therapeutic potential

Parkinson's disease (PD) is the second most common neurodegenerative disease caused by the progressive loss of dopaminergic neurons in the substantia nigra pars compacta. PD is characterized by motor dysfunctions as well as non-motor disorders. Orexin (also known as hypocretin) is a kind of neuropeptide involved in the regulation of motor control, the sleep/wake cycle, learning and memory, gastric motility and respiratory function. Several lines of evidence suggest that the orexinergic system is involved in the manifestations of PD, especially the non-motor disorders. Recent studies have revealed the protective actions and potential therapeutic applications of orexin in both cellular and animal models of PD. Here we present a brief overview of the involvement of the orexinergic system in PD, including the pathological changes in the lateral hypothalamus, the loss of orexinergic neurons and the fluctuation of orexin levels in CSF. Furthermore, we also review the neuroprotective effects of orexin in cellular and animal models of PD.